"Our main challenge is to now remove the nuclear fuel from the reactors," Takeshi Takahashi told visiting journalists on Monday. "Technically it's a very difficult problem, but we want to take it step by step."

Takahashi apologised repeatedly for the turmoil last year's accident at the plant had caused the people of Japan, and thanked the international community for its support.

More than 100,000 residents from the area have had to leave their homes, and are unsure when, or if, they can return.

The government announced in December that the plant had reached "cold shutdown" – a safe state achieved when temperatures inside the reactors stay below boiling point and radioactive leaks are kept to a minimum.

But fears rose this month that fuel in the No 2 reactor was heating up again, prompting the plant's operator, Tokyo Electric Power (Tepco) to inject more cooling water along with boric acid, which is used to prevent a nuclear chain reaction.

Asked repeatedly to explain the dramatic rise in temperature, Takahashi said the cause had been traced to a faulty thermometer, one of three inside the unit.

"The plant has reached a state of cold shutdown," he said. "We will now try to allow people to return to their homes as soon as possible."

Takahashi, who looked pale and exhausted, dismissed questions about his health. He took charge of the plant in December after his predecessor, Masao Yoshida, took early retirement to receive treatment for cancer of the oesophagus.

Tepco officials said Yoshida's diagnosis was not linked to his exposure to high radiation levels in the early days of the crisis when, he later admitted, he and his colleagues were convinced they would die.

Evidence of the damage inflicted on the ageing plant remains nearly a year on from the disaster.

Of the three reactors that went into meltdown, one is covered with tarpaulin and another appears intact, but the third is a mess of tangled metal. High radiation levels persist in areas close to the most badly damaged reactors.

About 100 new storage tanks, each capable of holding 1,000 tonnes of liquid, have been installed to store contaminated water from the reactor buildings. The water is then purified and used again to cool damaged fuel. The existing tanks will be full by April, Tepco said.

Yasuki Hibi, an engineer for a construction firm, said conditions at the plant had improved significantly in recent months, but added that workers were still limited to two three-hour shifts a day. "Radiation levels inside reactor No 3 are still too high for us to enter," he said.

About 3,000 people continue to work at the plant each day, according to Tepco. They monitor radiation, decontaminate workers and vehicles, prepare uniforms and equipment, and clear radioactive rubble and other material.

Kazuhiro Sakamoto, who hires workers and buys equipment for a Tepco affiliate, said: "The worst time was when radiation levels reached 250 millisieverts a year [a temporary upper exposure limit the government introduced soon after the accident].

"We couldn't find enough people to do the work, and we could only work in two-minute bursts."